Cast and extruded nylon are used in a wide variety of applications for their outstanding mechanical properties including high wear and abrasion resistance, superior strength and stiffness. Nylon's toughness, low coefficient of friction and wide size range availability make it an ideal replacement for a wide variety of materials from metal to rubber.

Standard nylon offers up to three times better wear than acetal and tops UHMW-PE in applications imposing high loads and stresses. Using nylon reduces lubrication requirements; eliminates galling, corrosion and pilferage problems; and improves wear resistance and sound dampening characteristics. Nylon has a proven record of outstanding service in a multitude of parts for such diverse fields as paper, textiles, electronics, construction, mining, metalworking, aircraft, food and material handling.

Many different types of nylon have been developed to satisfy a wide variety of application demands. Unmodified nylons are FDA, USDA and 3-A Dairy compliant for food contact applications. Nylons with added molybdenum disulfide offer tremendous value in general purpose structural or bearing and wear applications. Heat-stabilized nylons resist degradation at higher temperatures. And for demanding wear applications, an internally lubricated nylon may be specified.

The machining and fabrication guidelines in this article are applicable to most good-quality nylons falling into one of the generic classes described above. Glass-, mineral- or fiber-reinforced nylons will machine quite differently.

What follows is an introduction to some of the basic properties of nylon which may be relevant to machinists and fabricators, some specific guidelines for common machining operations and a handy troubleshooting guide.

Nylon Property Basics for Machinists and Fabricators
Despite the similarity to metals in fabrication equipment and typical applications, nylon and many other plastics have unique characteristics which must be considered when machining. Understanding these basic material characteristics is essential to the successful fabrication of precision parts from nylon stock shapes. Some important facts to keep in mind when machining nylon stock shapes:

• Plastics conduct heat more slowly than do metals. This means that heat generated due to pressure and friction during cutting operations tends to build up in the vicinity of the cutter, rather than being carried into the bulk of the part and ultimately away through a lathe chuck or fixture. With improper machining techniques, temperatures at the cutter interface can be quite high, even though much of the part remains cool to the touch.
• Thermal expansion is up to 10 times greater in plastics than metals. The coefficient of linear thermal expansion of nylon is typically about 50 x 10-6 inch/inch/°F, while that of steel (A36) is 6.3 x 10-6 inch/inch/°F. This means that a 1-inch-long steel bar will expand 0.00063 inch if its temperature is raised 100F. A 1-inch-long nylon bar would, under similar circumstances, expand 0.005 inch. Although this information is usually considered when designing parts for use at elevated temperatures, its relevance to holding tight tolerances when machining plastics should be evident in light of the preceding paragraph.
• Plastics are much more elastic and have lower operating temperatures than metals. They will distort or deflect more under pressure than metals. The elastic nature of plastics is important in many applications. For example, contact stresses in sheaves and gears made from nylon will be lower than in the corresponding metal parts carrying the same load due to the nylon's higher elasticity. This is one reason why plastics can often replace metals having much higher strength. To the fabricator, higher elasticity may mean lighter cuts and different fixturing techniques than would be used with metals in order to avoid distortion of the part during machining. Also, keep in mind that most plastics soften considerably at elevated temperatures. Nylon is about twice as pliable at 250F as it is at room temperature.
• Plastics are not as strong as most metals. Usually this fact will have been taken into account by the part designer. Machinists, however, need to recognize that sharp corners can concentrate stress and accelerate part failure. Sharp corners such as OD or ID transitions should be radiused, and proper thread cutting techniques are essential to reduce stress concentration and failure due to stress cracking. These practices, critical when using notch-sensitive materials, can contribute greatly to the longevity of nylon parts.
• Nylon is hygroscopic. It absorbs and releases moisture in response to changes in the humidity of its surroundings. Nylon can absorb up to 8 percent of its weight in water when immersed. If placed in a dry environment, absorbed water will be released. This is typically a very slow process. The rate will depend on conditions of temperature and humidity, and part geometry. Use of water-based coolants during machining will have little or no effect on the moisture content of the finished part. "Wet" nylon tends to be more elastic and flexible than dry nylon.
• As nylon absorbs moisture it swells. Changes in dimensions due to moisture absorption can be estimated as 0.003 inch/inch for each percent of moisture change. A 1-inch-long nylon bar machined at a moisture content of 2 percent would grow by 0.019 inch if immersed in water and allowed to reach its saturation point of 8 percent moisture. Nylon parts which must be closely toleranced for use in a wet environment may be soaked in hot water until saturated, then machined to final tolerance. The finished part can be stored under water until ready for use, or if stored dry, conditioned by soaking prior to use. Usually moisture absorption in nylon need not concern the machinist, however it can explain small dimensional changes due to changes in humidity which may be important in close tolerance parts.
• Machining operations can induce internal stress. High-quality nylon stock shapes are delivered with very low residual stress. Improper machining or removal of large amounts of material can create large internal stresses that can result in warping, ovality or other dimensional instabilities. Whenever possible, select a stock shape which minimizes the amount of material to be removed to make a finished part. In some cases, it may be advantageous to order custom size stock or consider a near net shape nylon casting. The effects of machined-in stress can be minimized by allowing a part to rest for several hours between machining operations. In rare cases, it may be necessary to post-machine anneal a nylon part if extraordinary dimensional stability is required.

Fabrication guidelines
The machining techniques detailed below were developed and refined by DSM Engineering Plastic Products. Fabricators may want to experiment with tool materials, tool angles, speeds and feeds to obtain optimum results.

General machining tips:

• Positive tool geometries with ground peripheries are suggested.
• Carbide-grade tooling with polished top surfaces should be used.
• Use adequate chip clearances to prevent clogging.
• Properly support the material to prevent it from springing away from the cutting tool.
• Coolants, though not required, may be used for optimum finishes or close tolerances. If coolants are used, a spray mist water soluble oil is suggested. Pressurized air or vacuum is commonly used for chip removal and as a light coolant.

Guidelines for turning nylon:

Depth of cut	Speed (fpm)	Feed (inch/rev)
0.150 in.	500-600	0.010-0.015
0.025	600-700	0.004-0.007

A fine-grained C-2 carbide is generally recommended for these operations.

Guidelines for drilling nylon:

Nominal hole diameter 1/16 1/8 1/4 1/2 3/4 1 in. 1 1/2 2 in. or larger

Feed (inch/rev) 0.007-0.015 0.007-0.015 0.007-0.015 0.015-0.025 0.015-0.025 0.020-0.050 0.020-0.050 0.020-0.050

With high-speed steel (M10, M7, M1), 150-200 fpm is suggested.

Important note: When drilling large diameter holes, a slow spiral (low helix) drill or general purpose drill ground to a point angle of 118° with a lip clearance of 9° to 15° is recommended. In both instances, the lip rake should be ground off, i.e., dubbed off, and the web thinned.

Drill a small, 1Ú2-inch maximum diameter hole at a speed of 600 to 1,000 rpm using a positive feed of approximately 0.006 inches per revolution. Avoid hand feeding the drill because grab can occur and stress or cracks may develop. A secondary drilling at a speed of 400 to 500 rpm is required to expand the hole to 1-inch diameter.

Guidelines for threading and tapping
Threading should be done by a single point using a carbide insert and taking four to five 0.001-inch passes at the end. For tapping, use the specified drill with a two-flute tap. Keep the tap clear of chip build up. Use of a coolant when threading or tapping is suggested.

Mike Oliveto, Jack Sharp and Kathy Bell of DSM Engineering Plastic Products assisted in the preparation of this article. All statements, technical information and recommendations contained in this article are represented in good faith, based on tests believed to be reliable and practical field experience. The reader is cautioned, however, that the authors cannot guarantee the accuracy or completeness of this information and it is the reader's responsibility to determine the suitability of this information in any given application.

Dr. David Rosenfeld is the manager of technical services at DSM Engineering Plastic Products Inc. Located in Reading, PA, the company manufactures a range of nylon products. For more information, Dr. Rosenfeld can be reached at (610) 320-6600.

John Raynor is the marketing manager for Charlotte, NC-based Piedmont Plastics. Established in 1968, Piedmont distributes and fabricates nylon as well as other types of materials. For more information, Raynor can be reached at (704) 597-8200.

Plastics Machining & Fabricating
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